Semantic Actions in YACC



7.2 Semantic Actions in YACC

TBoS p 156-169

Semantic actions are attached to grammar rules by following each rule by a :=. This YACC definition receives a list of as and changes them to bs.

  a  := [b | ];
  a := [b];)

(compile (function ) [a a a a a])
[b b b b b]

The first rule says that any input of the form a is to be translated into an output consisting of b consed to the translation of . The syntax of requires that the input be a non-empty list composed of occurrences of a. So (compile (function ) [a a a]) gives [b b b]. The second rule is the base case.

As in Shen, round brackets signify function applications and square ones form lists. The following reformulation is an example:

    := (question  );)

(define question
  NP VP -> (append [(protect Is) it true that] NP VP [?]))

(compile (function ) [the cat likes the dog])
[Is it true that the cat likes the dog ?]

and are both reserved non-terminals. always succeeds consuming none of the input and under semantic completion returns the empty list. always succeeds and consumes all of the input and under semantic completion returns that remaining input.

1. Introduction

2. License

3. History

4. The Core Language

4.1 Base Types
4.1.1 Symbols
4.1.2 Strings
4.1.3 Numbers
4.2 Function Applications
4.3 The Top Level
4.4 Arithmetic

4.6 Sequences

4.6.1 Lists
4.6.2 Tuples
4.6.3 Vectors

4.7 lambda and let
4.8 Global Assignments
4.9 Higher Order Functions
4.10 Lazy Evaluation
4.11 I/O
4.12 Loading Files
4.13 Streams
4.14 Exceptions
4.15 Hashing
4.16 Property Lists
4.17 Eval

5 Defining Functions

5.1 Partial Functions
5.2 List Handling Functions
5.3 String Handling Functions
5.4 Tuple Handling Functions
5.5 Vector Handling Functions
5.6 Guards
5.7 Backtracking
5.8 Writing in Kλ
5.9 Macros

6. Packages

7. Shen-YACC

7.1 Recognisor Generator
7.2 Semantic Actions

8. Shen Prolog

8.1 Sample Programs

9. Types

9.1 Types and Constructors
9.2 Functions and Types
9.3 Synonyms

10 Sequent Calculus

10.1 Recursive Types

10.2 Exotic Types

10.2.1 Dependent Types
10.2.2 Negative Types
10.2.3 Subtypes
10.2.4 The Type of All Sets

11 Glossary of Functions

12 The Syntax of Shen

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